1. Field of the Invention
The invention relates generally to toxic waste containment site management equipment and methods and specifically to tomographic equipment and methods that employ radiowaves to image a plume below a waste containment pit.
2. Description of the Prior Art
Hazardous byproducts of modern industry and radioactive wastes cannot be disposed of without due regard to the dangers associated with the waste. Such waste may remain hazardous for years and may be toxic in even small quantities. Therefore, such materials cannot be allowed to seep into soils or move through underground water aquifers because of the potential dangers to food and water supplies and plant, animal and fish environments. Very commonly such wastes are deposited in waste containment sites that are membrane-lined clay pits. Government regulations dictate that such sites be monitored for leakage, because numerous such sites have leaked and caused severe environmental damage. Such leakage takes the form of a plume, and comprises a toxic mixture of chemicals, therefore the term "toxic plume" has been coined to describe mobile underground leaks.
Monitoring wells encircling waste containment sites are often used to obtain water and soil samples for laboratory analysis and compliance with government regulations. Sensing a toxic plume flow is hit-or-miss, and depends upon the geology and hydrology of the underlying layers. Generally speaking, a monitoring well must intersect a toxic plume for the plume to be detected. Several such wells that intersect a toxic plume may be needed to judge a plume's velocity and the results of any mitigation efforts. For practical reasons, the number of wells must be limited, thus a plume may not ever contact any of the monitoring wells, and go undetected.
The underlying geology and hydrology of a waste containment site can be determined with substantially one hundred percent coverage with just a few monitoring wells located outside the immediate perimeter of a pit by employing radiowave tomography scanning. Both vertical and horizontal wells may be used to host a transmitting antenna in one well and a receiving antenna in an adjacent well. One or both antennas are moved successively to various points and a measurement of the intervening geology's attenuative affect and phase shift affect on the radio signals is taken. Tomography scanning and data processing can help to visually determine the electrical conductivity represented in each displayed pixel analogous an image plane.
U.S. Pat. No. 5,066,917, issued Nov. 19, 1991, to the present inventor, Larry G. Stolarczyk, describes a two borehole method for detecting an anomalous geological zone in a rock layer using radiowave scans to map conductivity changes in the rock layer. (See, the discussion related to FIG. 19.) A second higher frequency scan yields a tomography scan that includes both background conductivity changes and a diffraction shadow. Geologic noise is then eliminated from the tomography scan to net a tomographic image from the diffraction shadow due to the anomalous geological zone.
K. A. Dines and R. J. Lytle, in "Computerized Geophysical Tomography," Proceedings of the IEEE, Vol. 67, No. 7, July 1979, pp. 1065-1073, describe using computerized tomography techniques as an aid in geophysical exploration. Detailed pictures of the electromagnetic properties of the regions between a pair of boreholes can be reconstructed. Iterative solution techniques are used to solve large sets of linear equations related to the line integral data and remote observables.
Radiowaves propagating along subsurface paths are highly sensitive to the electrical parameters of a geologic medium, e.g., conductivity ((.sigma.), permittivity (.epsilon.), and magnetic permeability (.mu.). The attenuation rate (.alpha.) and phase (.beta.) constants of the propagating electromagnetic wave depend upon the electromagnetic parameters of the rock mass as: ##EQU1## where: .omega.=2.pi.d=radiating frequency (f in Hertz),
.mu.=.mu..sub.r .mu..sub.o =the magnetic permeability where .mu..sub.r is the relative permeability constant and .mu..sub.o is the absolute permeability of free space (4.pi..times.10.sup.-7 henry/meter), PA0 .epsilon.=.epsilon..sub.r .epsilon..sub.o =the electrical permittivity of the medium where .epsilon..sub.r is the relative permittivity constant and .epsilon..sub.o is the absolute permittivity of free space (1/36.pi..times.10.sup.-9 farad/meter), and .sigma.=the effective conductivity of the medium in Siemens/meter.
In non-magnetic rocks, the relative permeability is near unity. The conductivity ((.sigma.) is proportional to the porosity of the soil and the electrical conductivity of the pore space fluid. Equation (1) shows that the signal attenuation rate (.alpha.) in nepers (Np) per meter (Np=8.686 dB) and phase (.beta.) in radians per meter are proportional to the half power of the conductivity of the medium and the frequency of the radiowave. Radiowave imaging (RIM) may be used to acquire and process the measured data and to determine the contours of constant attenuation rate and phase across the image plane between drillholes. Equation (1) can be used to determine the conductivity distribution in an image plane between drillholes.
Radiowave tomography scanning can help construct representative images of the electrical conductivity of the site medium between sensing wells. Continuous wave (CW) monochromatic radio signals propagate over signal paths in the target geologic zone. In a conductive medium, only the geology within the first Fresnel ellipsoid significantly influences the conductivity of the zone (see, Hill, "Diffraction by a half-plane in a lossy medium," J. Applied Physics, Vol. 10, June 1991; and see, Parkhomenko, Electrical Properties of Rocks, Plenum Press, NY:1967, [illustrates the dependence of inverse conductivity in rocks]). Increasing the operating frequency decreases the minor and major axes of the ellipsoid. Increasing the operating frequency such that all of the radiowave energy is absorbed along the path between transmitter and receiver locations maximizes the resolution in the image.
The conductivity of a rock mass is dependent on the amount of water present, the water's conductivity, and the variation in porosity in the target zone. Full saturation is limited by the available porosity. In alluvium, clays and moisture react to increase the conductivity of the target zone. The conductivity of dry tuff approaches seventeen millisiemens/meter. When saturated with moisture, the conductivity increases to thirty-seven millisiemens/meter. Therefore, migrating liquid masses, such as toxic plumes, may be tomographically imaged.